<Distortion of Digital Signal>
There are known audio devices for processing sound signals, such as digital audio recorders, digital mixers, and effects units, for example. To such audio device, an analog sound signal is input via a microphone or an input terminal. The analog signal input to the audio device is amplified by an analog amplifier, followed by conversion to a digital signal. The amplification factor of the analog amplifier is set by a user by manually operating an operation unit provided in the audio device.
As illustrated in FIG. 7, an A/D converter of the audio device has the maximum voltage for A/D conversion (for example, “5.0 V”) defined in advance. When the amplification factor of the analog amplifier is too large relative to the signal level (that is, volume) of the analog signal input to the audio device, the signal level of the amplified analog signal sometimes exceeds the maximum voltage for A/D conversion (refer to a signal waveform on the left in FIG. 7). In such a case, the parts exceeding the maximum voltage for A/D conversion of the amplified analog signal are truncated without A/D conversion. As a result, the continuous analog signal is converted to a discontinuously distorted digital signal (refer to a signal waveform on the right in FIG. 7). The distortion of the digital signal becomes unrestorable noise. In recording and music performance, unexpected excessive input occurs often. By incorrectly setting the amplification factor of the analog amplifier, noise is generated when excessive input occurs.
<Limiter Circuit>
Some conventional audio devices are provided with a limiter circuit as a measure to the excessive input. As illustrated in FIG. 8, when excessive input occurs, the limiter circuit performs limiter processing to instantaneously reduce the amplification factor of the analog amplifier. As the conventional limiter circuit, there are those using, for example, a VCA (voltage controlled amplifier) or a photoresistor.
Such VCA is an amplifier having input and output of analog signals and input of the control voltage. The limiter circuit using the VCA outputs a control voltage in accordance with the signal level of the analog signal to the VCA. The VCA changes the amplification factor of the analog signal based on the input control voltage.
The photoresistor is a cadmium sulfide cell and changes the resistance value in accordance with the amount of received light. The limiter circuit using the photoresistor is provided with an LED as a light emitting unit. Such LED is turned on when excessive input occurs. The photoresistor decreases the resistance value by receiving light from the LED. The decrease in the resistance value of the photoresistor changes the amplification factor of the analog amplifier.
<Backup Function>
Some conventional digital audio recorders are provided with a backup function in which one analog signal is amplified by two amplification factors to be stored individually. The conventional backup function is achieved by providing two signal paths to input one analog signal. The two signal paths are connected to respective analog amplifiers. The analog amplifier connected to one of the signal paths amplifies the analog signal by one amplification factor, which is a value set by a user. The analog amplifier connected to the other signal path amplifies the analog signal by the other amplification factor, which is a value lower than the user's setting. The one amplification factor is set by a user by operating an amplification factor adjustment knob. Meanwhile, the other amplification factor is set by a user by calling a setting screen on a display panel by inputting a value on the setting screen.